The Mach-Zehnder interferometer is one of the most popular configurations for high resolution fibre optic sensors (FIG. 1). In this type of optical fibre sensor, light from coherent source 100 is injected into single mode fibre 101. This light is directed into two fibres by coupler 102; the reference fibre 103 and the signal fibre 104. The light from these two fibres is recombined by coupler 105 where optical interference takes place and is monitored by detectors 106 and 107. The signals at detectors are changed if the optical path length in one arm (the signal arm 104) of the interferometer changes with respect to the other (the reference arm 105). There are two basic ways of changing the path length; intrinsic or extrinsic. For an intrinsic fibre optic interferometer the signal fibre itself is stretched for example by piezoelectric cylinder 108 (or heated etc) to change the path length, so the light never has to leave the fibre. For the extrinsic fibre optic interferometer, light leaves the fibre, is collimated, passes through a measurement cell where optical path length changes and is then focussed back into the fibre. Typically, Mach-Zehnder interferometer sensors measure temperature, pressure, sound, acceleration, limited displacement, chemical species concentration etc.
It is usual to use a Michelson interferometer (FIG. 2) to measure displacements, particularly if the displacement is relatively large. Coherent light from laser 200 passes along single mode fibre 201 to coupler 202. Light travels along signal fibre 203, leaves the fibre at end 204 and is collimated by lens 205. This light subsequently is reflected by flat reflector 206 back to lens 205 and thence back into the core of fibre 203 at fibre end 204. This back reflected light interferes in coupler 202 with the reference light reflected from mirrored end 207 of fibre 208. The intensity resulting from the interference of these two beams in coupler 202 passes along fibre 209 and is detected by detector 210. In prior interferometers, the fibre-lens combination is moved as a unit relative to reflector 206 for example by piezoelectric stack 211 which consequently changes the optical path length in the signal arm.